Cardiac Cells and Action Potentials

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Questions and Answers

What is the primary function of the SA node in the heart?

To collect oxygenated blood from the lungs

To regulate blood pressure in the arteries

To serve as the natural pacemaker (correct)

To conduct electrical signals to the ventricles

Which type of cardiac action potential is associated with the SA and AV nodes?

Cardiac mechanical potential

Skeletal muscle potential

Slow action potential (correct)

Fast action potential

What is the approximate rate of impulses generated by the AV node?

60-80 beats/minute

20-40 beats/minute

80-100 beats/minute

40-60 beats/minute (correct)

Where is the SA node located within the heart?

In the superior posterolateral wall of the right atrium Signup and view all the answers

What mechanism causes ions to move across the membrane during the rapid action potential phase?

Ions move across membranes along concentration and electrical gradients Signup and view all the answers

What membrane potential is needed for the sodium channels to close after depolarization?

-90 mV Signup and view all the answers

What is the primary role of the sodium channels with high threshold for opening?

To allow the rapid depolarization of the action potential Signup and view all the answers

What characterizes the state changes of voltage-gated sodium channels?

They exhibit three states: open, closed, and inactive. Signup and view all the answers

What is required for the fast action potential of the ventricles to be generated?

A greater than threshold intensity of the stimulus Signup and view all the answers

What could happen if the sodium channels do not reactivate properly after repolarization?

The heart will experience arrhythmia. Signup and view all the answers

Which statement best describes the opening of sodium channels during depolarization?

Their opening is influenced by the change in membrane potential. Signup and view all the answers

What is the importance of returning to the resting membrane potential of -90 mV?

It prepares the membrane for subsequent action potentials. Signup and view all the answers

Which describes the nature of the stimulus intensity for opening fast sodium channels?

A specific greater than threshold intensity is necessary. Signup and view all the answers

What is the primary effect of increased parasympathetic activity on automaticity?

It decreases the phase 4 slope. Signup and view all the answers

What is the total delay in conduction from the SA node to the ventricular muscle?

0.16 seconds Signup and view all the answers

What physiological change occurs during the delay of conduction at the AV node?

The atria empty blood into the ventricles. Signup and view all the answers

Which of the following conditions decreases automaticity?

Hyperoxemia. Signup and view all the answers

What is the significance of the slow rise in resting membrane potential (RMP) before reaching threshold potential?

It prevents the heart from reaching action potential too quickly. Signup and view all the answers

What occurs when the resting membrane potential is depolarized to the threshold potential?

Sodium enters the myocyte Signup and view all the answers

What is the resting membrane potential of the myocyte as described?

-90 mV Signup and view all the answers

Which gate is closed at the resting stage of the sodium channels?

M gate Signup and view all the answers

What initiates the opening of the fast voltage-gated sodium channels?

Depolarization to about -65 mV Signup and view all the answers

During which phase do the sodium channels remain unresponsive to any stimulus?

Phase 0 until part of phase 2 Signup and view all the answers

What happens to the H gate of the sodium channels during the resting phase?

It is inactivated Signup and view all the answers

What configuration change occurs in sodium channels upon reaching the threshold potential?

M gate opens while H gate closes Signup and view all the answers

Which statement best describes the behavior of the sodium channels at resting membrane potential?

M gate is closed, H gate is open Signup and view all the answers

What triggers the activation of L-type calcium channels in pacemaker cells?

Reaching a threshold voltage of about -40 mV Signup and view all the answers

Which of the following statements is true regarding the resting membrane potential (RMP) of pacemaker cells?

It is less negative than other cardiac cells Signup and view all the answers

What is the primary mechanism behind the self-excitation of sinus nodal fibers?

Leakiness to sodium and calcium ions Signup and view all the answers

What characterizes the action potential generation in pacemaker cells?

Slow rise due to sodium and calcium influx Signup and view all the answers

Which feature is specifically associated with the automaticity of cardiac cells?

Ability to generate spontaneous action potentials Signup and view all the answers

What ionic event occurs during phase 4 diastolic depolarization in pacemaker cells?

Leakage of sodium and calcium through channels Signup and view all the answers

What is the effect of a high sodium concentration in the extracellular fluid (ECF) on pacemaker cells?

It facilitates leakage of sodium into the cells Signup and view all the answers

What is the significance of the threshold voltage of -40 mV in pacemaker cells?

It marks the activation point for calcium channels Signup and view all the answers

Study Notes

Types of Cardiac Cells

SA node functions as the natural pacemaker of the heart, initiating impulses at a rate of 60-80 beats per minute.
AV node acts as a secondary pacemaker with a slower rate of 40-60 beats per minute.

Types of Action Potentials

Fast action potentials occur in the atria, ventricles, and Purkinje fibers.
Slow action potentials are characteristic of SA and AV nodes.

SA Node Anatomy

Measures about 3 mm wide, 15 mm long, and 1 mm thick, located in the right atrium near the superior vena cava.

Fast Action Potential Mechanism

Involves rapid influx of sodium ions (Na⁺) through fast voltage-gated sodium channels after depolarization from -90 mV to approximately -65 mV.
Sodium channels transition through three states: closed (resting), open (activation), and inactivated.
Absolute refractory period occurs during phase 0 until partway through phase 1; no stimulus can activate channels during this time, necessitating higher thresholds for firing.

Ionic Basis for Fast Action Potential

Sodium ions move across the membrane down their concentration and electrical gradients.
L-type calcium channels activate when the membrane potential reaches about -40 mV, contributing to action potential generation.

Automaticity of Pacemaker Cells

Pacemaker cells can generate spontaneous action potentials without external stimuli, a trait particularly developed in the SA and AV nodes.
Automaticity results from leaky sodium and calcium channels leading to gradual depolarization.

Resting Membrane Potential (RMP)

RMP for pacemaker cells is around -55 to -60 mV, with a threshold for action potential generation at about -40 mV.

Conditions Affecting Automaticity

Increased parasympathetic activity (e.g., vagal stimulation) can decrease automaticity by negatively affecting phase 4 depolarization slope.
Hyperoxemia may also influence automaticity.

Delay in Conduction Pathway

Conduction time from the AV node to ventricular muscles is approximately 0.04 seconds, contributing to a total AV nodal delay of about 0.09 seconds.
The total conduction time from the SA node to ventricular muscle is around 0.16 seconds, allowing time for atrial blood to empty into the ventricles before contraction.

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Description

Explore the different types of cardiac cells, including the SA and AV nodes, and their role in heart rhythm. This quiz will help you understand the variation in action potentials within the heart's chambers and conduction system. Test your knowledge of cardiac physiology.